Studieson

The anticoagulant factor Protein S circulates in two major forms, free and bound to C4b binding protein (C4BP). We report a monoclonal antibody-based enzyme-linked im-munosorbent assay of human Protein S that is sensitive (detection limit 30 ng) and free of notable competition from other vitamin K-dependent proteins. We demon-strate assay sensitivity to Protein S-C4BP complex, as well as to free Protein S by (a) results of immunoaffinity chromatography involving the assay antibody; (b) paral-lelismbetween the standard curve and a plasmadilution curve, including dilutions>1 00-fold; (C) lack of effect of added C4BPon ProteinS values;and ( reactivity in the assay by high-molecular-massplasma ProteinS in gel permeationstudies. These latter studies also indicate that uncomplexedProteinS has an apparent molecular mas